1. Field of the Invention
The present invention relates to a transmitting and receiving apparatus wherein an antenna is shared by a transmission system and a receiving system, and more particularly to an automatic switching arrangement for interconnecting the transmission system, the receiving system, and the antenna.
2. Description of the Related Art
In the field of mobile communications, a transmitting and receiving apparatus is used in which a single antenna is shared by its transmitting and receiving systems. In this type of apparatus, a switching or duplexing method is employed for separating the transmitting and receiving systems to avoid mixing the transmitted and received signals.
FIG. 7 is a schematic diagram of a conventional transmitting and receiving apparatus including digital communication equipment using the time division multiple access (TDMA) and time division duplex (TDD) systems. In this transmitting and receiving apparatus generally indicated by 20, a changeover switch 22 is disposed in the feed line to a single antenna 21, and is connected to a transmitter 23 and a receiver 24, whereby the antenna 21 can be connected to either the transmitter 23 or the receiver 24 through the switch 22. Also, an isolator 25 is connected between the switch 22 and the transmitter 23 for preventing the generation of intermodulation (IM) due to reflection of a transmitted signal as well as for avoiding unstable operation of a power amplifier (PA) caused by load fluctuations.
FIG. 8 is a circuit diagram of the changeover switch 22. The switch 22 includes coupling capacitors C1 to C3 and a high-frequency short-circuiting capacitor C4. A 1/4-wavelength high-impedance line L1 is short-circuited at one end by the capacitor C4 at high frequencies, thus making the impedance of the input terminal of switching power viewed from the other end P1 infinite and resulting in the input terminal being in an open state at high frequencies. The switch 22 also has diodes D1 and D2 and a 1/4-wavelength stripline L2 having characteristic impedance ZO which is equal to a circuit impedance, generally 50 .OMEGA..
An explanation will now be given of the transmitting and receiving operation of the changeover switch 22. In the transmitting operation, a switching current is allowed to flow through the switch 22 so as to turn on the diodes D1 and D2. This causes a point P3 to be short-circuited to ground through the diode D2, whereby a portion of the circuit viewed from a point P2 at one end of the stripline L2 toward the receiving circuit has an infinite impedance and is in an open state at high frequencies. On the other hand, since the points P1 and P2 are short-circuited to each other through the diode D1, a transmitted signal from the transmitting circuit is radiated from the antenna 21. This transmitted signal is partially absorbed in the isolator 25, thereby preventing the IM generation and unstable PA operations.
In a receiving operation, the switching current is discontinued to turn off the diodes D1 and D2. Accordingly, the points P1 and P2 are disconnected from each other by the diode D1, and the point P3 is disconnected from ground by the diode D2. Thus, a received signal input through the antenna 21 can be completely conducted to the receiving circuit.
However, the transmitting and receiving apparatus of the above conventional type requires a battery for providing a switching current in order to switch between the transmitting and receiving operations and also requires a current control circuit. This deteriorates the power efficiency of the transmitting and receiving apparatus and also shortens battery life. Further, the provision of a current control circuit inevitably enlarges the overall apparatus and increases its cost. There is also a problem of low reliability in the performance of the apparatus, for example, since any erroneous operation of the current control circuit renders the apparatus inoperable.
A measure that has been considered but has not yet been published or patented, to overcome the above-described drawbacks inherent in the conventional apparatuses, may be the use of a changeover switch 30 as illustrated in FIG. 9 for automatically switching between the transmitting and receiving systems depending on an output voltage of a transmitting signal. The switch 30 includes diodes D3 to D6 and a 1/4-wavelength stripline L3 having characteristic: impedance Z0. This switch 30 takes advantage of the fact that the transmitting power is greater than the receiving power. For example, in the Personal Handyphone System (PHS), the peak receiving power is several dozens of .mu.W, while the transmitting power is approximately 100 mW.
The transmitting and receiving operation of the switch circuit of FIG. 9 will now be explained in connection with its application to the PHS system by way of example.
During a transmitting operation, a high-frequency signal having approximately 100 mW of power is outputted. By the use of diodes that are turned on by 10 mW of power, a, the diodes D3 to D6, one of the diodes D3 and D4 or one of the diodes D5 and D6 is accordingly turned on when the transmitted signal exceeds 10 mW. Which diodes are on, from among diodes D3 or D4, and D5 or D6, depends on the polarity of the voltage: a positive voltage turns on the diodes D3 and D5, while a negative voltage turns on the diodes D4 and D6. At this time, a branch point P4 and the terminal of the transmitting circuit are electrically connected through the diodes D3 and D4 at high-frequencies. A point PX is short-circuited to ground through the diodes D5 and D6 so that the impedance of a portion viewed from the point P4 to the receiving circuit can be infinite, thus resulting in this portion being in an open state. Hence, a transmitted signal is mostly radiated from the antenna, and partially reflected by the antenna and absorbed in the isolator before returning to the transmitting circuit.
On the other hand, during a receiving operation, all the diodes D3 to D6 are turned off, since the receiving power inputted from the antenna is only several dozens of .mu.W. Thus the point P4 is disconnected from the terminal of the transmitting circuit by the diodes D3 and D4 and also the point P5 is disconnected from ground by the diodes D5 and D6. Accordingly, a received signal inputted from the antenna can be completely sent to the receiving circuit.
In this manner, the configuration of the changeover switch 30 makes it possible to eliminate the need for the provision of a battery and a current control circuit for providing a switching current. This improves the power efficiency of the transmitting and receiving apparatus, thereby avoiding an increase in the size and cost of the apparatus, as well as enhancing the reliability of its performance.
Diodes have different signal passing characteristics depending on input power; in general, a larger amount of power passing through a diode decreases its insertion loss (I. L.). In the foregoing conventional changeover switch 22, however, since a large amount of power is supplied from a battery, the I. L. is substantially constant regardless of the power of a transmitted signal. However, if automatic switching is performed according to the power of the transmitted signal, as in the changeover switch 30, the I. L. is susceptible to varying, and more specifically, a smaller amount of power increases the I. L., thereby causing a degradation in the performance of the overall apparatus.
Moreover, whichever switch 22 or 30 is used, a signal passes through the switch during a transmitting operation, which increases losses in the transmission system and further degrades the power efficiency. A switch having high isolation characteristics is also required for switching between the transmitting and receiving systems, thereby increasing the cost of parts.